REEVALUATING TAPHONOMIC WINDOWS: EXCEPTIONAL PRESERVATION PATHWAYS IN THE WAUKESHA AND THEIR CONNECTION TO OTHER LAGERSTÄTTEN

The non-biomineralizing fossil record can provide insights into paleoecology and biodiversity that their shelly counterparts cannot match, but the variety of preservation pathways experienced can make comparing deposits difficult. Co-occurring pathways related in geologic time and/or paleoenvironment are sometimes grouped into “windows” or “types,” which can facilitate comparisons. But what can be done when the window closes? The Burgess Shale-type (BST), where organisms were preserved primarily as carbonaceous compressions in shelfal marine environments, was only an open taphonomic window from the Late Ediacaran to the Early Ordovician. Many shallower deposits from the later Paleozoic resemble BST preservation, but this resemblance has rarely been investigated.

This study presents on the taphonomic pathways seen in one such deposit: the Silurian (Llandoverian) Waukesha lagerstätte, Wisconsin. This laterally restricted soft-bodied fossil deposit represents a clay-rich hydrodynamic trap in the intertidal to subtidal zone of a carbonate shelf. Both biomineralized and soft-bodied taxa are found at this site, although biomineralized material, like trilobites and conulariids, have been largely demineralized or remineralized. Soft-bodied forms are preserved as carbonaceous compressions with secondary phosphatization as francolite. Phosphatization is at least partly taxonomically controlled, and may occur as a thin coat or as three-dimensional replacement. 3-D replacement is most notable in a small megacheiran, where legs, muscle blocks, and even likely nerve cords may be preserved.

Thus, despite their different depositional environments, the Waukesha preserves similar organisms by comparable preservation pathways to typical BST deposits. Yet, the Waukesha also shows some similarity to much younger Mesozoic stagnation deposits. It may be more useful to think of soft-bodied lagerstätten in terms of histologically- and paleoenvironmentally-based continuua rather than as discrete “windows.” This will help connect lagerstätten across time and depositional environments.